Magma source region size controls along axis variations in crustal thickness, axial depth and relief at plate spreading centers

Abstract

The amplitude of variations in axial depth and across-axis relief that define spreading center segmentation vary systematically with spreading rate. Seismic and gravity data show that crustal thickness also varies along segments. We derive a highly idealized model to relate axial lithospheric thickness, crustal thickness, and effective magma source region size to segment-scale variation in axial depth and across-axis relief. All magma to build the crust is assumed to pool in a segment-centered magma source region and is distributed along the spreading axis by flow in dikes. The magma source and dike are regarded as a closed system and the magma pressure is taken to decrease linearly with the volume of magma going into dikes. The pressure-volume relation driving lateral magma flow is taken to be proportional to the along-axis gradient of depth. Axial depth variations depend on: (1) local isostatic compensation from crustal thickness variations; and (2) across-axis relief or axial valley depth. The axial valley depth is related to crustal and lithospheric thickness using the recently published model of Liu and Buck (2018). Excellent fits to bathymetric data for 12 segments from intermediate to ultra-slow spreading ridges are found by varying three model parameters (HC0, HL and ΔP/ΔV). The dependence of pressure drop on volume withdrawn is parameterized in terms of an effective radius for a spherical source region in an elastic half-space. A key result is that the effective magma source radius increases with spreading rate.